Electron optical device



March 8, 1938. H- JQHANNSON 2,110,669

ELECTRON OPTICAL DEVICE Filed July 3, 1935 lm/en'or' Helmu'f doh Tmorl Patented Mar. 8, 1938 UNITED STATES PATENT OFFICE ELECTRON OPTICAL DEVICE Helmut Johannsen, Berlin-Charlottenburg, Germany, assigner to General Electric Company, a corporation of New York Application July 3, 1935, Serial No. 29,755 In Germany July 4, 1934 2 Claims. (Cl. Z50-27.5)

The present invention relates to improvements which represents diagrammatically a four-elecin electron-optical devices and is directed more trode microscope which may suitably be employed particularly to the elimination of distortion and in the practice of my invention. aberration effects obtaining in the use of such Referring to the structure illustrated, I pro- 54 devices. vide a surface i which may act as an electron 5 It has been found that when an electron beam beam source. This. may alternatively comprise a discharge generated directly from a cathode) cathode or other inherently electron-emissive elemember, or indirectly from a target member, is ment or a target adapted to give off electrons drawn through an apertured diaphragm held at When bombarded from an electron source.

lo a potential suiiicient to develop a definite electro- Spaced from the cathode i at a distance c varil static eld, a diffusion or refraction of the beam able within limits of from about .5 to 1.5 millitakes place. If the electrostatic field employed meters is a focusing electrode or diaphragm 2 is properly placed and suitably adjusted with reprovided With a Central aperture 3- An epprOprilation to the velocity of the discharge, this diffuate size for the aperture I have found to be in the l5 sion or refraction of the electron beam may be neighherhOOd 0f one millimeter. 15 employed to obtain eiects comparable to those About one-half millimeter from the main achieved by passing light rays through lenticular focusing grid 2 I provide a second electrode which systems. Thus, if the refracted rays be pr0 is also illustrated as an apertured diaphragm or jected on a screen member located adjacent a grid 4. This diaphragm, WhlCh COHSl'nteS o0 highly positive electrode member, a magnified means fOr mdifying the electrostatic field adi 20 image of the cathode surface, or the target surjacent the electrode 2, may suitably be of dimeniace, as the case may be, will be produced theresions comparable with those of that electrode. on. By this means it is theoretically possible to Approximately one millimeter 0r less from the obtain magniiications as high as 1 to 10,000. second electrode is mounted a third apertured In the absence of special precautionary measeleetlOde 0r anode 5 Which is Capable 0f being 25 ures there is nothing to insure that the distorting brought to a relatively high positive potential effect of the electrostatic diaphragm employed With respect t0 the emitting sOilrCe Behind will be such as to produce a perfectly magnified the enede 5 and spaced et a Considerable dis image. Indeed, it has been found that in the tance therefrom I have shown a projecting screen conventional system employing as electrodes a, 6 of iiuorescent material suitable for receiving '30 cathode and two additional electrodes formed as and reprOduCing the magnified image 0f the diaphragms the images reproduced show consid- SOUrCe i- Wiih Other dimensions Corresponding erable distortion. This is partly manifested in to those given above I have found it convenient an unnatural picture curvature and partly in a t0 use a Sperling between the screen and the 0bblurring of the outer edge of the image. In a, ject being observed which is in the neighborhood 35 sense, this latter aspect may be compared with the of 240 millimeters. It should be understood, how- Well-known phenomenon of spherical aberration ever, that this dimensiOn as Well as thOse previof optical lenses, wherein the difference in relaously Stated are merely eXemplary 0f a particular tive curvatures between the center and the edge arrangement Which I haVe found practical in a0- of the lens results in a plurality of displaced tual tests. Any of the values given maybe varied 40 focusing points and a vague and distorted image. Within wide limits provided a reasonable pro- In accordance with my present invention one portionality is maintained which will result in or more additional grid diaphragms are provided proper focusing of the eorpuselllar rays intermediate the focusing aperture and the screen In the operation of an electron microscope to enhance the quality of the image formed. I constructed in accordance with my invention the 45 have found that by regulating the electrostatic anode 5 is made liighli7 pOSiiiVe With respeCif i0 potential of the auxiliary diaphragm both the the object source l. In this Way the electrons magnification and the range of sharp definition generated are eeeeleraied t0 pass at high Veleeity of the image may be greatly increased. through the aperture 3 of the diaphragm 2. If

The novel features which I consider characthe potential of this diaphragm is then lowered, 50 teristic of my invention are set forth with parthat is, made less positive than the anode or ticularity in the appended claims. The invention made negative with reference to that of the source itself, however, will best be understood by referelectrode I, a focusing effect will take place quite ence to the following specification taken in conlike that occurring when light rays are passed nection with the drawing, the single figure of through a lens system. Finally, by varying the 55 UJI potential of the auxiliary electrode 4 and bringing it to an optimum value the electrostatic field adjacent the focusing electrode may be so modified that a maximum of amplification is obtained coupled with the highest possible picture quality. This optimum value will generally lie between the potential of the electrode l and that of the anode being in most instances of the order of magnitude of the latter and accordingly positive with respect to the focusing electrode 2.

Actual experiments have shown that a fourelectrode microscope embodying my invention may be made to yield magnification 25 per cent higher than has been found possible with threeelectrode assemblies. Furthermore, the introduction of a second grid permits an increase in the diameter of the sharply defined picture surface of about 100 per cent. Changes in these factors, formerly obtainable only by varying the relative spacing of the various electrodes, can now be obtained simply by regulating the potential impressed upon the auxiliary electrode. Additional improvements in reproduction can be accomplished by special formations of the focusing electrodes, such as surrounding the apertures by funnel-like projections.

It is also possible and my invention contemplates that the corrective deformation of the potential eld can be brought about by the superposition of a magnetic field on the electrostatic field created by the grid electrodes.

While I have shown a particular embodiment of my invention, it will of course be understood that do not wish to be limited thereto since many modications in the structure may be made, and I contemplate by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:-

1. An electronic magnification device comprising an electron beam source, means for reproducing an enlarged image of the source, and a series of relatively thin closely spaced electro-des interposed between the said source and imagereproducing means and each having a substantially circular opening therein arranged to be traversed by the electron beam, said electrodes including an anode for accelerating the beam toward such means, a focusing electrode between the anode and the source, said focusing electrode being effective when negatively charged with respect to the anode to produce substantial magnification of the beam image on the image-reproducing means, and an intermediate electrode between the focusing electrode and the anode, said intermediate electrode being effective when positively charged with respect to the focusing electrode to increase the region of undistorted magnication of the beam image.

2. An electronic magnification device comprising an electron beam source, a fiuorescent screen or the like adapted to reproduce an enlarged image of the source and a series of closely spaced apertured diaphragins positioned between the source and the screen, said diaphragms comprising an anode adapted to be maintained at a high positive potential with respect to the beam source, a focusing electrode between the anode and the source, said focusing electrode being effective when negatively charged with respect to the anode to produce substantial magnification of the beam image on the screen, and an intermediate electrode between the focusing electrode and the anode, said intermediate electrode being effective when positively charged with respect to the focusing electrode to increase the region of undistorted magnification of the beam image.

HELMUT JOHANNSON. 

